1. Field of the Invention
The present invention relates to a projection lens that enlarges and projects, for example, display information from a light valve, such as a transmissive or reflective liquid crystal display device or a DMD (digital micro-mirror device), and more particularly, to a projection lens applicable for a so-called front projection display device, and a projection display device using the same.
2. Description of the Related Art
In recent years, projection display devices using light valves, such as a liquid crystal display device or a DMD display device, have come into widespread use. In particular, a projection display device has been widely used which uses three light valves corresponding to illumination light components of three primary colors, such as R, G, and B, to modulate the illumination light components, combines the light components modulated by the three light valves using, for example, a prism, and displays an image on a screen using a projection lens.
In the projection display device that uses a color composition optical system to compose the modulated light components from the three light valves and projects the composed light, the projection lens needs to have a large back focal length in order to arrange a prism for color composition. In addition, the spectral characteristics of the color composition optical system vary depending on the angle of incident light. Therefore, the projection lens needs to have characteristics that an entrance pupil is positioned at a sufficiently long distance as viewed from the reduction side. That is, the projection lens needs to have telecentricity. The projection lens also needs to correct aberration according to the resolution of a device.
As the size of the light valve is reduced, the precision thereof is improved, and personal computers are widely spread, the use of the projection display device for presentation is increased. Therefore, there is an increasing demand for a projection display device having a high performance, a high brightness, and a small size.
However, as the performance and brightness of the projection lens are increased, the size of the lens is increased, and the size of a moving mechanism for moving a lens group to adjust focus is also increased.
An inner focus type in which only some lenses in a lens group can be moved to easily adjust focus has been known in the field of an imaging lens of a single-lens reflex camera or video camera. However, when this inner focus type is applied to a projection lens without any change, an emission angle is small, and it is difficult to ensure telecentricity.
Therefore, in the projection display device using the inner focus lens, when a color composition prism is used to perform color composition, color unevenness occurs on a screen. Therefore, it is difficult to use the inner focus lens according to the related art as a projection lens of a projector, without changing the structure of the inner focus lens.
Therefore, in order to solve the above-mentioned problems, JP-A-2001-166205 filed discloses a projection lens and a projector apparatus.
That is, the projection lens disclosed in JP-A-2001-166205 includes a first negative lens group, a second positive lens group, and a third positive lens group arranged in this order from a magnification side, and a lens arranged closest to a reduction side in the first lens group is moved along the optical axis X to adjust focus. In addition, the gap between the first lens group and the second lens group, and the focal length of the entire lens system are set in a predetermined range.
The projection lens having the above-mentioned structure has an appropriate back focal length, telecentricity, a high optical performance capable of reducing distortion and chromatic aberration, a simple inner focus structure, and a small size.
However, the projection lens disclosed in JP-A-2001-166205 is slower than F2.0.
In recent years, there is an increasing demand for a projection lens that is faster than F1.95 and can project a clear projection image onto a screen even when a projection display device is placed in an indoor environment in the daytime.